Distribution, historical development and ecophysiological attributes of oak species in the eastern United States

Approximately 30 Quercus species occur in the eastern United States, of which Q. alba, Q. rubra, Q. velutina, Q. coccinea, Q. stellata and Q. prinus are among the most dominant. Quercus distribution greatly increased at the beginning of the Holocene epoch (10 000 years BP), but has exhibited major changes since European settlement in the 18th and 19th centuries. Prior to European settlement Quercus grew and regenerated in uneven-aged conditions. At the times oak growth was very slow (<1.0 mm/year) for long periods. Quercus species exhibited continuous recruitment into the canopy during the 17th, 18th and 19th centuries, but stopped recruiting in the early 20th century. Since that time, later successional, mixed-mesophytic species have dominated understory and canopy recruitment, which coincides with the period of fire exclusion. Major oak replacement species include Acer rubrum, A. saccharum, Prunus erotina. Logging of oak forests that have understories dominated by later successional species often accelerates the oak replacement process. Oaks typically have low tolerance for current understory conditions, despite the fact that they produce a large seed with the potential to produce an initially large seedling. Oak seedlings in shaded understories generally grow very slowly and have recurring shoot dieback, although they have relatively high net photosynthesis and low respiration rates compared to many of their understory competitors. Without severe competition from non-oak tree species, oaks should have the physiological capability for long-term survival beneath their own canopies in uneven-age.